1. Field of Invention
The present invention relates to methods of producing composite structures, and more particularly to methods of producing composite structures directed by surface roughness and to composite structures made by the methods.
2. Discussion of Related Art
One of the primary goals of synthetic colloidal chemistry is to create new kinds of particles that have a wide variety of shapes and functionalities and overall sizes in the range from a few microns to a few nanometers. The dominant approach taken by many groups worldwide is through bottom-up synthesis, including “self-assembly” (Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418) of atomic, molecular, and supramolecular components. Self-assembled structures can be simple, such as spheres (Antl, L.; Goodwin, J. L.; Hill, R. D.; Ottewill, R. H.; Owens, S. M.; Papworth, S.; Waters, J. A. Colloid Surf 1986, 17, 67), disks (Mason, T. G. Phys. Rev. E 2002, 66, 60402), platelets (van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature 2000, 406, 868), and cubes (Murphy, C. Science 2002, 298, 2139). They can also be more complex, such as tetrapods (Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787), clusters (Manoharam, V. N.; Elsesser, M. T.; Pine, D. J. Science 2003, 301, 483), liposome-microtubule complexes (Raviv, U.; Needleman, D. J.; Li, Y.; Miller, H. P.; Wilson, L.; Safinya, C. R. Proc. Nat. Acad. Sci. 2005, 102, 11167), and colloidosomes (Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Baush, A. R.; Weitz, D. A. Science 2002, 298, 1006). Random thermal forces cause colloidal particles to diffuse rapidly in a liquid regardless of their structures; this Brownian motion can overcome gravity and keep the particles dispersed homogenously over long times (Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge Univ. Press: Cambridge, 1989). Despite the increasing sophistication of self-assembly approaches, including multi-step procedures, that have produced a rich variety of new structures (van Blaaderen, A. Nature 2006, 439, 545), no universal recipe exists for creating monodisperse colloids that have arbitrarily prescribed shapes and sizes using bottom-up approaches.
One of the current inventors has demonstrated that high-throughput automated stepper lithography can be used to generate bulk dispersions of an enormous range of desirable particle shapes having exquisite fidelity in the colloidal length scale range (See PCT/US2007/018365 filed Aug. 17, 2007 assigned to the same assignee as the current application, the entire contents of which are incorporated herein by reference). These stepper-produced lithographic particles, or “LithoParticles”, can be designed to have a virtually limitless diversity of crisp monodisperse shapes that have structures well below 10 microns in lateral dimensions. By contrast to most micro-electromechanical systems (MEMS) (Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press: Boca Raton, 2002) applications, in which the desired structures remain attached to the wafer's surface, in our application, the LithoParticle structures, created by UV-exposure and development, are completely liberated from the wafer's surface by total lift-off into a liquid. The wafers merely serve as re-useable flat substrates for making the particles. As a demonstration of the power of this approach, we have designed and fabricated “colloidal alphabet soup”: a dispersion of microscale polymer LithoParticles representing all twenty-six letters of the English alphabet in a viscous liquid. Moreover, we demonstrated control over the color and internal composition of the LithoParticle letters by incorporating red, green, and blue fluorescent dyes into them. By successively coating more than one layer of resist and exposing using different masks, we have built up complex three-dimensional LithoParticles that are not limited to simple slab-like shapes and that can be used to create complex layered internal compositions. These advances represent major breakthroughs in designing model colloids over top-down efforts in micromachining (Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, O. Appl. Phys. Lett. 1994, 64, 2209; Brown A. B. D.; Smith, C. G.; Rennie, A. R. Phys. Rev. E 2000, 62, 951; Sullivan, M.; Zhao, K.; Harrison, C.; Austin, R. H.; Megens, M.; Hollingsworth, A.; Russel, W. B.; Cheng, Z.; Mason, T. G.; Chaikin, P. M. J. Phys. Condens. Matter 2003, 15, S11) and imprintation (Rolland, J. P.; Maynor, B. W.; Euliss, L. E.; Exner, A. E.; Denison, G. M.; DeSimone, J. M. J. Am. Chem. Soc. 2005, 127, 10096) work to which we have contributed.
We can produce high-fidelity polymeric LithoParticles by the following versatile method. Polished wafers are spin-coated first with a sacrificial layer of water-soluble polymer and subsequently with a layer of UV-sensitive photoresist [FIG. 1(a)]. The thickness of the resist layer can be controlled from about 100 nm to many microns with excellent uniformity over the entire wafer's surface (Madou, M. J. Fundamentals of Microfabrication: The Science of Miniaturization, 2nd ed.; CRC Press: Boca Raton, 2002). A high-throughput, robotically-automated, ultraviolet (UV), lithographic projection exposure system, or “stepper”, is used to rapidly expose the photoresist at sub-micron feature-sizes (Id.). The spin-coated wafers are exposed with the mask patterns of letters or other shapes on a photomask at 5× reduction using an Ultratech i-line stepper. This exposure causes cross-linking of the polymer resist [FIG. 1(b)]. An organic developer removes the unexposed resist without dissolving the sacrificial layer; as a result, the sacrificial layer is covered with a dense array of particles [FIG. 1(c)]. By dissolving the water-soluble sacrificial layer, the particles are lifted completely off of the surface into aqueous solution [FIG. 1(d)]. Surface charges on the particles inhibit aggregation for pH >8. Once in solution, methods of surface chemistry and solvent exchange can be used to further stabilize the particles and change the liquid from aqueous to non-aqueous (Antl, L.; Goodwin, J. L.; Hill, R. D.; Ottewill, R. H.; Owens, S. M.; Papworth, S.; Waters, J. A. Colloid Surf 1986, 17, 67).
The details of our synthetic procedure are as follows. LithoParticles are produced by successively spin-coating two uniform layers onto five-inch polished diameter silicon wafers. Water-soluble Omnicoat (Microchem) is initially spun onto each wafer at 3,000 rpm to produce a sacrificial layer of 0.3 μm. Next, SU-8 2001 epoxy resist in cyclopentanone is laden with red, green, and blue fluorescent dyes by adding 0.0015 g each of either Nile Red, NBD-X, or 2,6-ANS (Invitrogen) to 15 mL of SU-8 and mixing thoroughly. Each of the SU-8 solutions is then spin-coated over the sacrificial Omnicoat layers at 3,000 rpm, producing a resist layer thickness of 1.0 μm. By contrast to some photoresists, crosslinked SU-8 exhibits little optical absorption in the visible spectrum and has good chemical resistance. The density of the SU-8 after baking and removing the solvent is 1.24 g/cm3. A reticle-mask (Toppan) that contains all 26 letter of the English alphabet in high density (area fraction of approximately 50%) is created by electron beam lithography (MEBES) using a layout designed in L-Edit software (Tanner EDA). The lateral dimensions of a letter on the reticle are about 35 μm×20 μm. The wafers are then exposed to 365 nm light, using an Ultratech 2145 i-line stepper (5× reduction, 0.35 micron feature size, wavelength 365 nm) with an automated wafer handling system capable of 60 wafers/hour, at a power of 233 mJ/cm2 (optimized for feature fidelity). The lateral dimensions of the printed letters after 5× reduction by the stepper are 7 μm×4 μm with only a 1 μm effective “pen-width” (i.e. the width of the stroke defining the letters). We have obtained sub-micron pen-widths for other shapes using a different mask. The wafers are developed first by organic SU-8 developer using mild agitation, and then are lifted off of the wafer surfaces into aqueous solution using water-based Omnicoat developer. After lift-off, the particles are stable against aggregation in basic solutions but can begin to aggregate if the pH is changed to be acidic. Surface functionalization and surface modification chemistry, if desired, is performed on the particle surfaces at this stage, typically immediately after lift-off before any irreversible aggregation or clumping occurs. After producing three separate bottles of monodisperse red, green, and blue fluorescent microscale letters, we mix them together to form multi-colored fluorescent colloidal alphabet soup of LithoParticles in water suitable for multi-line excitation and detection using confocal microscopy.
Probing the thermally-driven dynamics of systems of many interacting particles that have interesting and varied non-spherical shapes in three-dimensions may help unlock the mysteries of self-assembly. Since fast 3-d confocal microscopy is an excellent tool for studying colloidal model systems, we have developed dispersions of colloidal LithoParticles that are compatible with confocal microscopy by incorporating red, green, and blue fluorescent dyes into the resist prior to spin-coating. We have optimized the concentrations of the dyes to provide bright fluorescence, yet these concentrations are low enough that we can still adjust the stepper's exposure to provide the necessary cross-linking that maintains the mechanical integrity of the particles. We have created separate aqueous solutions of red, green, and blue fluorescent microletters; after combining these solutions, we have used multi-wavelength excitation and detection laser scanning confocal microscopy (TCS SP2 AOBS laser-scanning microscope: Leica) with a 63× oil-immersion objective (NA 1.40) to obtain an image section of red, green, and blue colloidal alphabet soup [FIG. 2(a)]. After thresholding and cross-filtering the red, green, and blue channels, a composite image is created. Although some concepts of shape-dependent assembly of non-spherical disk-shaped particles driven by depletion attractions have been previously introduced to make columnar aggregates of microscale disks (Mason, T. G. Phys. Rev. E 2002, 66, 60402), a non-obvious application of depletion attractions with LithoParticles can be used to create columnar assemblies of crosses in which the arms of the crosses are highly aligned, interdigitated and angular offset aggregation of crosses, columnar tubes comprised of square donuts, lock-and-key aggregation caused by the insertions of an arm of a cross into the hole of a donut, and the formation of a cup by causing a donut to aggregate face-to-face with a cross [FIGS. 2(b)-(e)]. In all these examples, the small spheres used as a depletion agent, which create the depletion attraction between the larger objects, are nanoemulsion droplets (Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635) having an average radius of 57±5 nm at a droplet volume fraction of 0.1.
Due to the high resolution of the stepper, the control over the shapes of the letters is exquisite. The effective width of the pen that has written the letters is about 1.0 μm, as we intended. Scanning electron microscope (SEM) images of the letters [FIG. 3(a)] reveal a striking crispness and high degree of uniformity: the polydispersities in lengths and in thicknesses of the letters are less than 10%. These SEM images are obtained by drying particles on a Ni/Cu tape (Ted Pella Inc.) and sputtering 5 nm of gold using a Hummer 6.2 Sputterer (Anatech Ltd.) to reduce charging. Images are acquired using a field emission JEOL JSM-6700F SEM at a working distance of 8 mm and at 10 kV and 10 μA. Jamming (Liu, A. J.; Nagel, S. R. Nature 1998, 396, 21) and interlocking of the deposited particles is evident. Indeed, the interlocking of particles that have “arms” and “hooks” in a disordered structure is actually an unusual non-equilibrium jammed state that can support tension; simpler particle shapes, such as spheres (Torquato, S.; Truskett, T. M.; Debenedetti, P. G. Phys. Rev. Lett. 2000, 84, 2064) and ellipsoids (Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikin, P. M. Science 2003, 303, 990), which can only support compression, repulsively jam but do not interlock. In addition to letters, we have designed a variety of smaller particle shapes based on a single layer: square donuts (toroidal particles with a hole), square crosses (particles with four arms in the same plane), triangular prisms, and pentagonal prisms [see FIGS. 3(b)-(e)]. The Brownian motion of these particles in solution is very noticeable. By adapting stabilization and solvent exchange procedures (Yethiraj, A.; van Blaaderen, A. Nature 2003, 421, 513), one can match both the refractive index and the density of these particles in organic solvents.
The robotic automation of the stepper's exposure can be used to rapidly mass-produce bulk dispersions of LithoParticles. Five-inch wafers, exposed at one wafer per minute, yield roughly a quarter of a billion particles per minute, permitting the production of bulk dispersions. This production rate far surpasses that of other top-down methods, such as continuous-flow lithography (Dendukuri, D.; Pregibon, D. C.; Colloins, J.; Hatton, T. A.; Doyle, P. S. Nature Mater. 2006, 5, 365), which has reported rates of a hundred particles per second. The stepper provides massively parallel high throughput while maintaining superior alignment and exposure fidelity arising from a mechanically stable platform. By incorporating all of the equipment for making LithoParticles into a robotically automated track system, a continuous rate of 108 particles per minute or more can be achieved.
Beyond incorporating fluorescent dyes into single-layer particles, we have achieved a high level of control over the internal composition of the particles and have achieved complex multi-layer structures. For instance, we have produced both fluorescent and magnetically-responsive LithoParticles by incorporating a variety of organic dyes and organically-coated nanoparticles, such as iron-oxide, into the photoresist layer prior to exposure and crosslinking. In addition, we have created complex 3-d multilayer LithoParticles by coating and exposing layers in succession using a set of reticles that represent the cross-sections of desired 3-d shape at different heights. As a simple example, we show hybrid bilayer Janus LithoParticles that have been created by exposing a lower resist layer laden with blue fluorescent dye with the square cross pattern, and then coating a second layer of resist containing red dye, aligning, and exposing with the triangular pattern [FIG. 4]. A microscopic dark field alignment system (μ-DFAS) is used by a piezoelectric feedback system to align the previously exposed wafer with the new reticle. Holographic laser tweezers can also be used to create complex 3-d colloidal structures (Korda, P.; Spalding, G. C.; Dufresne, E. R.; Grier, D. G. Rev. Sci. Instr. 2002, 73, 1956), yet the throughput and edge fidelity of the stepper-based method is much higher. Using our approach, deep-UV steppers with sub-100 nm feature sizes, could produce colloidal particles having dimensions smaller than 1 μm×0.6 μm×0.1 μm.
The high-throughput production of customizable colloidal LithoParticles by automated stepper technology may open doors for many new research directions. One very exciting area is to use confocal microscopy to study the process of thermally-driven self-assembly of differently shaped components that have controlled interactions (Frenkel, D. Nature Mater. 2006, 5, 85). In essence, we can use the power of top-down lithography to generate model dispersions of monodisperse colloidal LithoParticles, for example, that can enable us to study and understand the science of bottom-up self-assembly. This problem lies at the heart of understanding structure-function relationships in molecular biology, for example. In order for thermally driven self-assembly to occur in a reasonable time, Brownian motion of the components should be significant. Larger particles generated using other lithographic methods are not truly colloidal and may not be useful to explore self-assembly, because the extremely slow diffusion of larger structures make self-assembly studies impractical. By contrast, our single-layer and multi-layer fluorescent LithoParticles are small enough that dynamics, such as phase transitions and self-assembly, can be explored in three dimensions using fast confocal microscopy (Gasser, U.; Weeks, E. R.; Schofield, A.; Pusey, P. N.; Weitz, D. A. Science 2001, 292, 258). Indeed, deep-UV steppers could produce sub-micron LithoParticles that would diffuse and self-assemble even more rapidly.
In addition to providing model systems of complex shapes, LithoParticles can be used in a wide range of other applications. In cell biology, LithoParticles could serve as novel fluorescent probes that may be customized and adapted to study dynamic changes of microstructures inside cells. LithoParticles that have tailored surface functionalization, such as charge groups (Leunissen, M. E.; Christova, C. G.; Hynninen, A. P.; Royall, P.; Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A. Nature 2005, 438, 235) or DNA (Tkachenko, A. V. Phys. Rev. Lett. 2002, 89, 148303), could provide new and interesting possibilities for creating artificial interacting architectures (Frenkel, D. Nature Mater. 2006, 5, 85). Single component or multicomponent model systems of LithoParticles can be used to explore the equilibrium phase behavior and phase transitions of mixtures of non-spherical shapes (Adams, M.; Dogic, Z.; Keller, S. L.; Fraden, S. Nature 1998, 393, 349), providing a better understanding of the fundamental science of liquid crystals. LithoParticles that have arms, whether straight or curved, can jam and interlock to form unique colloidal glasses (Weeks, E. R.; Weitz, D. A. Phys. Rev. Lett. 2002, 89, 095704). By manipulating LithoParticles with laser tweezers (Cheng, Z.; Chaikin, P. M.; Mason, T. G. Phys. Rev. Lett. 2002, 89, 108303), one may be able to mark cells with a desired shape or letter. Combining top-down LithoParticles with bottom-up synthetic approaches, such as those used to form colloidosomes (Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Baush, A. R.; Weitz, D. A. Science 2002, 298, 1006) and dense clusters (Manoharam, V. N.; Elsesser, M. T.; Pine, D. J. Science 2003, 301, 483), would offer an even greater range of interesting and potentially useful morphologies.
The advent of multi-layer monodisperse LithoParticles, including Janus LithoParticles, which have feature sizes around one micron or less, represents an important advance in lithographic colloidal dispersions. Colloidal LithoParticles exhibit significant Brownian motion and provide building blocks suitable for use in thermodynamic self-assembly driven by selective interactions, such as depletion attractions, and thermal diffusion. We have demonstrated controlled production of toroidal particles and shape-specific lock-and-key assembly. Moreover, single-layer and multi-layer particles can be made from a great range of materials, including organic, inorganic, and metallic materials.
Entropic depletion attractions have been used to cause aggregation of simple particulate dispersions (Y. N. Ohshima et al., Phys. Rev. Lett. 78, 3963 (1997); D. Rudhardt, C. Bechinger, and P. Leiderer, Phys. Rev. Lett. 81, 1330 (1998); S. Asakura, and F. Oosawa, J. Chem. Phys. 22, 1255 (1954); S. Asakura, and F. Oosawa, J. Polym. Sci. 33, 183 (1958)). More recently, it has been possible to create a variety of dispersions of particles having custom-designed shapes (C. J. Hernandez, and T. G. Mason, J. Phys. Chem. C 111, 4477 (2007); S. Badaire et al., J. Am. Chem. Soc. 129, 40 (2007); D. Dendukurl et al., Nature Mater. 5, 365 (2006); J. P. Rolland et al., J. Am. Chem. Soc. 127, 10096 (2005); M. Sullivan et al., J. Phys. Condens. Matter 15, s11 (2003); J. C. Love et al., Langmuir 17, 6005 (2001); A. B. D. Brown, C. G. Smith, and A. R. Rennie, Phys. Rev. E 62, 951 (2000); M. D. Hoover, J. Aerosol Sci. 21, 569 (1990); L. Manna et al., Nature Mater. 2, 382 (2003)). When combined with particles having complex shapes, depletion attractions provide a promising route for creating complex colloidal assemblies (T. G. Mason, Phys. Rev. E 66, 060402 (2002)). Nanoscale colloids, known as depletion agents, can induce depletion attractions between larger non-spherical particles to create complex equilibrium phases (M. Adams et al., Nature 393, 349 (1998)), shape-dependent aggregation (T. G. Mason, Phys. Rev. E 66, 060402 (2002)), and multi-step hierarchical assembly dynamics (T. G. Mason, Phys. Rev. E 66, 060402 (2002); Z. Dogic, Phys. Rev. Lett. 91, 165701 (2003)) in solution. Entropic depletion attractions between colloidal particles are ubiquitous and arise solely from physical considerations of excluded volume. Larger colloidal particles dispersed in a liquid can aggregate when a sufficient concentration of a smaller depletion agent is added (S. Asakura, and F. Oosawa, J. Chem. Phys. 22, 1255 (1954); S. Asakura, and F. Oosawa, J. Polym. Sci. 33, 183 (1958)). As both larger and smaller colloids diffuse in the liquid, the smaller colloids exert an osmotic pressure, Π, on the surfaces of the larger particles. When two larger particles nearly touch, the smaller colloids can become excluded from the region in between them, creating an attractive force due to an imbalance Π. This attractive force is very short in range, corresponding to the diameter, d, of the depletion agent. For large enough volume fractions, φs, of the smaller depletion agent, the maximum depth of the potential energy well can become significantly larger than thermal energy, kBT, leading to slippery diffusion-limited aggregation and even gelation of the larger colloids (J. N. Wilking et al., Phys. Rev. Lett. 96, 015501 (2006); A. D. Dinsmore et al., Phys. Rev. Lett. 96, 185502 (2006)). For smooth, spherical colloids, there is good agreement between the classic theoretical predictions and experiments (Y. N. Ohshima et al., Phys. Rev. Lett. 78, 3963 (1997); D. Rudhardt, C. Bechinger, and P. Leiderer, Phys. Rev. Lett. 81, 1330 (1998)). However, there remains a need for improved methods of assembling objects dispersed in a fluid including using surface roughness to direct the assembly of the objects.